Influence of falling height on the excitability of the soleus H-reflex during drop-jumps

The stretch-shortening cycle (SSC) is characterized by stretching of the target muscle (eccentric phase) prior to a subsequent shortening in the concentric phase. Stretch reflexes in the eccentric phase were argued to influence the performance of short lasting SSCs. In drop-jumps, the short latency component of the stretch reflex (SLR) was shown to increase with falling height. However, in jumps from excessive heights, the SLR was diminished. So far, it is unclear whether the modulation of the SLR relies on spinal mechanisms or on an altered fusimotor drive. The present study aimed to assess the spinal excitability of the soleus Ia afferent pathway at SLR during jumps from low height (LH - 31 cm) and excessive height (EH - 76 cm).

In 20 healthy subjects (age 25 +/- 3 years), H-reflexes were timed to occur at the peak of the SLR during drop-jumps from LH and EH.

H-reflexes were significantly smaller at EH than at LH (P < 0.05). Neither soleus and tibialis anterior background EMG nor the size of the maximum M-wave changed with falling height.

Differences in the H-reflex between EH and LH indicate that spinal mechanisms are involved in the modulation of the SLR. A decreased excitability of the H-reflex pathway at EH compared with LH is argued to serve as a 'prevention strategy' to protect the tendomuscular system from potential injuries caused by the high load. It is argued that pre-synaptic inhibition of Ia afferents is most likely responsible for the change in H-reflex excitability between the two jump conditions.